Building upon Zimbra

Hi,

Am new to this forum and even to the product(Zimbra). I have zimbra opensource edition got installed and running on my Linux machine. Am going through the code to understand it and the task before me is to add chat application upon zimbra. Will be so thankful for your help & suggestions to me regarding
How to analyse the source code of zimbra(OpenSourceEdition)
How can i proceed to add new features on existing zimbra application